GSM (Global System for Mobile communications) is a standard for mobile telephony. GSM differs significantly from its predecessors in that both signaling and speech channels are digital, which means that it is seen as a second generation (2G) mobile phone system. GSM is an open standard which is developed by the 3GPP.
As the GSM standard continues to develop, it retains backward compatibility with the original GSM mobile telephones; for example, packet data capabilities were added in the Release '97 version of the standard, by means of General Packet Radio Service (GPRS). Higher speed data transmission has also been introduced with Enhanced Data rates for GSM Evolution (EDGE) in the Release '99 version of the standard.
In release 7 of the 3GPP specifications a downlink dual carrier is introduced. The downlink dual carrier GSM EDGE Radio Access Network (GERAN) makes it possible for the mobile station to receive data on two different frequencies simultaneously. The reason for adding a new downlink carrier is to increase the peak throughput for Enhanced GPRS (EGPRS). The second carrier enables the mobile station to receive twice as many time slots as with a conventional Enhanced GPRS (EGPRS) system.
The GSM standard is a cellular radio network standard. A cell is an area served by a base station to which a mobile station connects. When the mobile station travels outside the area served by a particular base station, the mobile station must be served by another base station serving another, adjacent cell. In order for the system to handle the transfer from one cell to another a handover procedure is defined for each GSM system.
The handover procedure is a critical phase for the end-user in a GSM network. The handover procedure is used to control the mobile stations in a GSM network. The handover procedure measures the signal strength of a current serving cell and at least one surrounding cell. The results of these measurements are then reported back to the network. If the current serving cell is determined to be inferior to another cell, the target cell, the network initiates a handover from the current serving cell to the target cell, which upon completion of the handover becomes the new serving cell.
The handover procedure is typically initiated when the signal strength in the serving cell is worse than that of another adjacent cell. At this time it is not uncommon that the quality on the speech channel has degraded and that there can be problems for a control signal indicating a change of cell to reach the mobile station. It is necessary for all signaling in a handover procedure to reach the designated mobile station in order for the connection to continue. If the mobile station has traveled out of range for the signaling from the serving cell the connection will be terminated and the call will be dropped once the mobile station travels from the serving cell and is out of reach for the base station of the serving cell. A lot of efforts are therefore spent in a radio network to ensure that this signaling is successful to as large extent as possible.
The handover procedure becomes even more critical in networks dimensioned for speech version 3. The robustness of the speech and the associated control channels are unbalanced for this speech version. This will not result in any major problems as long as it is used to enhance the speech quality. However, if it is used to extend the service area for a cell the likelihood of dropped calls due to signaling failure on the associated control channels is increased as described above.
Thus, there is a problem in existing GSM networks when the MS has moved too far into a target cell before the handover command is sent. In that case it might be very hard for the MS to successfully decode it, which in turn will lead to a call being dropped. This is especially a problem were low AMR modes are used to extend the service area for Circuit Switched (CS) speech.
Hence, there exist a need for a method and a system that is able to increase the robustness of the handover procedure in a GSM network. A more stable handover procedure would also make the frequency planning less critical in non-AMR networks.